ECHINODERMA. OPHIUROIDEA 113 



typical case the followiug parts are found : — Encasing the 

 arms there are four rows of plates (fig. 44), one dorsal, one 

 ventral, and two lateral. The lateral plates are provided 

 with rows of spines. Between the lateral and the ventral 

 plates are apertures for the 

 passage of the tube-feet. The 

 greater part of the space en- 

 closed by the four rows of 

 plates is filled up by a longi- b 



tudinal row of ossicles (d) ; J = 



each ossicle consists of two 

 parts which are nearly always Fi f n %hiS°^»» t 



fused together along the dorsal plate ; b, lateral plate ; 

 I- -• i t t-< i c ? ventral plate; d, ambulacra! 



median vertical line. Each ossicles fused along the median 



Ossicle in the row articulates vertical line ; e, ambulacral 



groove. Enlarged, 

 with the preceding and suc- 

 ceeding one, so that the arm is flexible. On the uncler- 

 surface of the row there is a groove (e) in which the 

 water- vascular vessel and the nerve cord are placed; this 

 corresponds to the ambulacral groove of the star-fish, and 

 the plates above represent the ambulacral ossicles. 



The disc is usually round or pentagonal. On its 

 oral surface (fig. 45 A) the arms are prolonged so as to 

 reach the mouth. The last pair of ambulacral ossicles, — 

 those next the mouth, instead of being fused like the 

 others, remain free, and each ossicle unites with that 

 of the adjoining arm. The parts of the disc between 

 the arms are known as the interbrachial areas ; these are 

 covered mainly with scaly plates and granules, but in each 

 of the areas there is one large plate, the buccal plate (b), 

 and other smaller plates, projecting into the mouth, giving 

 it a stellate form. One of the buccal plates serves as the 

 madreporic plate. The genital openings (g) also occur on 



W. r. 8 



